The present invention relates to a chromatographic optical detector.
Chromatography is a separation process whereby individual chemical compounds which were originally present in a mixture are resolved or distinguished from one another. In some chromatographic applications, an optical detector can be utilized. In order to properly initialize prior art optical detectors utilized in chromatography applications, a logarithmic ratio (log ratio) circuit takes the log ratio of a first reference signal and a second chromatographic signal, which is typically representative of the absorption of a chemical solution. The reference signal is derived by splitting off and detecting a fixed proportion of the light source which serves to correct for changes in light intensity. The output of the log ratio circuit is an output signal representative of the log ratio of the first and second signals. The prior art typically requires some form of DC voltage or current signal which is subtracted from the output of the log ratio circuit to provide an initial zero value. This approach consequently requires a DC voltage or current circuit in conjunction with the log ratio circuit, thereby increasing the cost and complexity of the optical detector. By initially forcing the output voltage to zero, it is implied that the working liquid in the detector has zero absorption.
With the introduction of the material being analyzed, the working liquid becomes more absorbing, and thus the output from the log ratio circuit becomes a measure of sample concentration. Without this automatic zeroing feature, a large constant offset error would exist due to the difficulty of providing a reference signal equal to the initial chromatographic signal by mechanical/optical/chemical means. This constant offset is typically very large compared with the chromatographic signal.
Log ratio circuits typically include a pair of matched transistors having similar current and voltage characteristics. The log ratio transistors must conform accurately to the true voltage/current curves, as any departure results in a further offset error when the temperature varies.
This usually requires that the transistors, even though matched, must be placed within an oven for proper operating conditions, which necessarily further increases the cost of the overall optical detector circuit. Also, temperature variance problems are still encountered with such prior art approaches.
The DC reference voltage or current circuit and oven requirements are characteristics of prior art approaches for chromatographic optical detectors which increase the overall cost and complexity of such detectors.